In recent years, for the purpose of reducing weight and enhancing safety of an automobile, an increase in the strength of automobile components and materials used therein has been made, and with regard to steel sheets which are representative materials for the automobile components, a rate of use of a high-strength steel sheet has been increased. In order to achieve the reduction in weight while enhancing safety, it is necessary to increase a collision energy absorbing ability while increasing the strength. For example, it is effective to increase a yield stress of a steel material; and thereby, a collision energy can be absorbed efficiently with a low deformation amount. In particular, as a material used in the vicinity of a cabin of an automobile, materials having high yield stresses are widely used because there is a need to block a colliding object invading the cabin from the point of view of occupant protection. Particularly, the demand for a high-strength steel sheet having a tensile strength in a range of 590 MPa or more, and a high-strength steel sheet having a tensile strength in a range of 780 MPa or more has been increasing.
In general, as methods of increasing a yield stress, there are (1) a method of work-hardening a steel sheet by performing cold rolling, (2) a method of forming a microstructure including a low-temperature transformation phase (bainite or martensite) having a high dislocation density as a main phase, (3) a method of performing precipitation strengthening by adding microalloying elements, and (4) a method of adding solid-solution strengthening elements such as Si and the like. Among them, with regard to the methods (1) and (2), the dislocation density in the microstructure is increased; and thereby, workability during press forming is deteriorated drastically. This results in further deterioration of press formability of a high-strength steel sheet which originally has insufficient in workability. On the other hand, in the method (4) of performing solid-solution strengthening, the absolute value of a strengthening amount is limited; and therefore, it is difficult to increase the yield strength to a sufficient extent. Accordingly, in order to efficiently increase the yield stress while obtaining high workability, it is preferable that microalloying elements such as Nb, Ti, Mo, and V are added to perform precipitation strengthening of alloy carbonitrides for achieving a high yield stress.
From the above viewpoint, a high-strength hot-rolled steel sheet in which precipitation strengthening of microalloying elements is utilized has been put to practical use. However, the high-strength hot-rolled steel sheet in which the precipitation strengthening is utilized mainly has two problems. One is fatigue properties and the other is rust prevention.
With regard to the fatigue properties as the first problem, in the high-strength hot-rolled steel sheet in which precipitation strengthening is utilized, there is a phenomenon in which a fatigue strength is reduced due to softening of the surface layer of the steel sheet. In the surface of the steel sheet which directly comes into contact with a rolling roll during hot rolling, the temperature of only the surface of the steel sheet is reduced due to a heat releasing effect of the roll which comes into contact with the steel sheet. When the temperature of the outermost layer of the steel sheet falls below an Ar3 point, coarsening of the microstructure and precipitates occur; and thereby, the outermost layer of the steel sheet is softened. This is the main factor of the deterioration of the fatigue strength. In general, a fatigue strength of a steel material is increased as the outermost layer of the steel sheet is hardened. Therefore, in a high-tensile hot-rolled steel sheet in which precipitation strengthening is utilized, it is difficult to obtain a high fatigue strength at present. On the other hand, the purpose of increasing the strength of a steel sheet is to reduce the weight of an automobile body; however, the sheet thickness cannot be reduced in the case where the fatigue strength ratio is reduced while the strength of the steel sheet is increased. From this point of view, it is preferable that the fatigue strength ratio be in a range of 0.45 or more, and even in the hot-rolled high-tensile steel sheet, it is preferable that the tensile strength and the fatigue strength be maintained at high values with a good balance. Here, the fatigue strength ratio is a value obtained by dividing the fatigue strength of a steel sheet by the tensile strength. In general, there is a tendency that a fatigue strength increases as a tensile strength increases. However, in a material with higher strength, the fatigue strength ratio is reduced. Therefore, even though a steel sheet having a high tensile strength is used, since the fatigue strength is not increased, there may be a case where a reduction in the weight of the automobile body which is the purpose of increasing strength cannot be realized.
The other problem is rust prevention. Typically, as a steel sheet used in a chassis frame for an automobile, a cold-rolled steel sheet produced by cold rolling and annealing thereafter and an alloyed hot-dip galvanized steel sheet are not used, but a hot-rolled steel sheet having a relatively thick thickness in a range of 2.0 mm or more is mainly used. In the vicinity of a chassis where a paint on the surface of the steel sheet is easily peeled off due to physical contact with curbs, flying stones, or the like, a material having a thicker thickness than that required from a design stress is selected to be used in consideration of a corrosion thickness reduction amount (amount of reduced sheet thickness due to corrosion) during a service life; and thereby, the quality is guaranteed. Therefore, with regard to the chassis frame and the like, the reduction in weight by substituting the material to a high-strength steel sheet is delayed at present, compared to body components. Since the sheet thickness is thick as one of the characteristics of chassis components, arc welding is mainly conducted for welding the components. Since the arc welding has a higher heat input amount than that of spot welding, HAZ softening is more likely to occur. In order to obtain properties of being resistant to HAZ softening, precipitation strengthening by an addition of microalloying elements is mainly utilized. Therefore, it is difficult to apply a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having high rust prevention properties because annealing is conducted after cold rolling for the purpose of structure strengthening in the manufacture of these galvanized steel sheets. The reason that the precipitation strengthening by an addition of microalloying elements cannot be utilized for the steel sheet produced by performing annealing after cold rolling is described as follows. Even in the case where a hot-rolled steel sheet into which microalloying elements are added is subjected to a cold rolling at a high cold rolling rate (for example, 30% or higher) and then annealing is conducted at a temperature in a range of an A3 point or less, the microalloying elements suppress recovery and recrystallization of ferrite. Therefore, a microstructure is work-hardened in a state of being cold-rolled; and as a result, workability is deteriorated drastically. On the other hand, in the case where heating is performed at a temperature in a range of the A3 point or higher, precipitates coarsen; and as a result, there is a problem in that a sufficient increase in the yield strength is not obtained. Therefore, the precipitation strengthening by the addition of microalloying elements cannot be utilized.
As a hot-dip galvanized steel sheet which includes a hot-rolled steel sheet, Patent Document 1 discloses a method of producing a hot-dip galvanized steel sheet having a tensile strength in a range of 38 to 50 kgf/mm2. With regard to the steel sheet having such a strength level, a desired strength level is obtained without utilizing precipitation strengthening due to an addition of microalloying elements. However, methods of producing a high-strength steel sheet, a hot-dipped steel sheet, and an alloyed hot-dipped steel sheet, which have excellent collision properties and fatigue strength with a strength in a strength level of 590 MPa or more are not disclosed yet.